Conventionally, optical grating devices, such as linear scales or encoders, are measured using methods, such as scanning probe method (SPM) and optical diffraction effect (OD) by laser light sources.
The scanning probe methods such as Scanning Electron Microscopy (SEM), Atomic Force Microscope (AFM) are complex and require specific conditions for operation. For example, SEM can observe and measure the grating pattern in nano-scale but it is destructive test method. In addition, vacuum is required for operation of SEM. This method is not suitable to implement as an online test method and cannot measure a large linear scale. For another example, AFM can measure a small area usually under 100 umλ100 um but cannot measure large linear scale.
For the optical diffraction methods, the pitch of the grating device can be determined by the aid of laser diffraction in Littrow condition. This method can measure pitch of grating scale very precisely. However, this method utilizes laser as its light source and cannot determine exactly which pitch has error.
For the foregoing reasons, there is a need for an apparatus which can measure pattern of grating devices in different sizes or length, effectively, efficiently, and flexibly.